1. Field of the Invention
The present invention relates to the field of fuel gauging for monitoring battery charge or battery state of charge.
2. Prior Art
Fuel-gauging technology can both predict and increase battery life, by monitoring the battery characteristics and state of charge, and making adjustments to the charging parameters as needed. One of the important input data needed for most fuel-gauge algorithms is an accurate sampling of battery current. The high accuracy required for the battery current measurement, and the need to measure the battery charge and discharge current under all conditions, has previously required an extra sense element inside the battery charge and discharge paths. This extra sense element, typically a small resistor, adds power loss during both battery charging and discharging, proportional to the voltage and current through the battery current sense element.
FIG. 1 shows the foregoing prior art. A current sense element, typically a resistor as shown by Rsense, is connected in series with the battery pack. This current sense element is then used to measure battery current by converting the voltage drop across the sense resistor to a digital representation. Typically a thermistor RTHM is also provided to sense the temperature of the battery, both for more accurate prediction of battery performance and for fault detection, which reading is also digitized. The information is then provided to the fuel-gauging algorithm.
Typical battery packs for cell phones, laptop computers and the like will include over current protection which will protect against excessive charging and discharging currents. For this purpose, of course, battery current needs to be measured, and accordingly a typical battery pack will include a current sense resistor in series with the actual battery, plus two series connected MOSFETs having an opposite body connection so as to be able to turn off either an excessive charging current or an excessive discharging current. In addition, charging current is generally sensed by an external current sense resistor in series with another transistor switch coupled to the output of a switching regulator. All of these devices must be capable of conducting the maximum currents allowed, and accordingly are both relatively large, and in the case of the sense resistors, relatively expensive because of their capacity and accuracy requirements. Further, of course, that many switches and sense resistors in series with particularly the charger is a significant source of heating and power loss, and for those devices actually packaged in the battery pack, the voltage drop across the switches and sense resistor will require a higher battery voltage before system shutdown, thereby limiting the discharge voltage to a higher voltage and thus the useful capacity of the battery to a lower capacity.